The aim of the proposed research is to study individual differences in cognitive and personality development in relation to maturational rates at adolescence in the context of a neuropsychological model of hemispheric specialization of function. The study is intended to replicate previous findings and to extend them further in order to answer questions they raised. The data will be collected in a longitudinal design with an initial sampling of 80 pre-pubertal boys and 80 pre-pubertal girls from a middle-class community. They will be retested two years later, when most of them would be expected to show at least initial signs of puberty, thus making it possible to discriminate between early and late maturing individuals. The psychological battery will include measures of (a) differential abilities; (b) cognitive processes involved in the performance of tasks shown to be related to maturation rates; personal behaviors. Measures of maturation will include (a) neuromotor development and (b) physical development. Data analysis will emphasize relationships among measures as well as relationships between psychological functioning and rate of maturation prior to the onset of puberty and at puberty.